Several multi-access scenarios will exist in the near future, for example UMTS (Universal Mobile Telecommunication System)/GSM (Global System for Mobile communication) and UMTS/WLAN (Wireless Local Area Network). In many cases, the same end-user service can be carried by more than one of the included accesses. For example, best effort packet data can be carried by both UMTS and WLAN. A selection of which access to use must thus be done.
In the prior art, it has been observed that the accesses are, on a by call or session basis, not equally efficient for different types of end user services. Consequently, to increase the common multi-access capacity, it has been proposed to allocate calls or sessions of a service to the access by which the service is most efficiently handled. This is described in patent application nr: PCT/SE03/00611.
In Alcatel, “Simultaneous Access”, S2-030215, 3GPP TSG-SA WG meeting #29, 3GPP, it is discussed to enable simultaneous packet switched associations over WLAN and cellular (GPRS (General Packet Radio Service)). That is to establish and maintain two connection paths enabling packet transmission through both accesses without prior signaling.
There are two widely discussed ways to combine the existing cellular and WLAN structures, tight interworking and loose interworking. This is described in “WLAN-GPRS Integration for Next-Generation Mobile Data Networks” by Apostolis et al. IEEE Wireless Csommunication, October 2002.
In “Stand-Alone Data-Cells for UMTS”, Tdoc FEW-0032/01, 3GPP Future Evolution Workshop, October 2001 Helsinki, Source: Nortel Networks, an additional suggestion, WLAN integrated in UTRAN, is described.
In the above scenarios, there is a possibility to maintain associations through both cellular and WLAN radio access, as discussed in “Simultaneous Access”. The protocol level where the association shall be maintained will be dependent on the splitting point on the network side, i.e. the “last” common node for the different systems. Examples are:                For WLAN integrated in UTRAN (UMTS Terrestrial Radio Access Network) the splitting point for WLAN/UMTS is in the RNC (Radio Network Controller). Two associations are then maintained on RNC-level.        For tight interworking the splitting point is in SGSN (Serving GPRS Support Node). Two associations are then maintained on IP-level. That means two IP-addresses and multihoming, see “Simultaneous Access”.        For loose interworking the splitting point is in an IP-router. Two associations are then maintained on IP-level as for tight interwork.        
For all scenarios there is also a splitting point in the user terminals, i.e. the terminals need to be able to communicate through the different access systems and the terminal has to choose, or be instructed to choose, one of the systems.
End user services are partly characterized by a set of traffic characteristics, including packet size statistics. In “On the Need for Efficiency in the 802.11 QoS Solution” by M. Sherman, IEEE 802.11-01/045, January 2001, it is disclosed that the spectrum efficiency of the WLAN (IEEE 802.11) depends on the packet size. Small packets are inefficiently handled, especially for the higher order modulation schemes.